During the reinstallation, you'll be prompted whether you want to restore the file or not:

Configuration file `/etc/apparmor.d/usr.bin.firefox'
==> Deleted (by you or by a script) since installation.
==> Package distributor has shipped an updated version.
What would you like to do about it ? Your options are:
Y or I : install the package maintainer's version
N or O : keep your currently-installed version
D : show the differences between the versions
Z : start a shell to examine the situation
The default action is to keep your current version.
*** usr.bin.firefox (Y/I/N/O/D/Z) [default=N] ?